Optical interference, that modifies the transmitted and reflected intensities of light, occurs with the superposition of two or more beams of light. The principle of superposition states that the resultant amplitude is the sum of the amplitudes of the individual beams. The brilliant colors, for example, which may be seen when light is reflected from a soap bubble or from a thin layer of oil floating on water are produced by interference effects between two trains of light waves. The light waves are reflected at opposite surfaces of the thin film of soap solution or oil.
More importantly, a practical application for interference effects in thin films involves the production of coated optical surfaces. When a film of a transparent substance is deposited on transparent substrate such as glass, for example, with a refractive index which is properly specified relative to the refractive index of the glass and with a thickness which is one quarter of a particular wavelength of light in the film, the reflection of that wavelength of light from the glass surface can be almost completely suppressed. The light which would otherwise be reflected is not absorbed by a non-reflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted.
Considerable improvements have been achieved in the anti-reflective performance of such films by using a composite film having two or more superimposed layers. Two different materials may be used in fabricating such a composite film, one with a relatively high index of refraction and the other with a relatively low index of refraction. The two materials are alternately deposited to predetermined thicknesses to obtain desired optical characteristics for the film. In theory, it is possible with this approach to design multi-layer interference coatings for a great variety of transmission and reflection spectrums. This has led to the development of many new optical devices making use of complex spectral filter structures. Anti-reflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band rejection filters are some of the examples of useful devices employing thin film interference coatings.
One particular type of interference coating is the bandpass filter, which is designed to allow wavelengths within a predetermined range of the desired pass-band to be transmitted, while a range of wavelengths on either side of the pass band are highly reflected. Ideally a bandpass filters should be square in its response; thus, the transition from the rejection regions to the passband should be as rapid as possible, or expressed differently, the slope or transition region should be as steep as possible, while obtaining a pass band region that is uniform having little or no ripple.
Multi-cavity filters have been manufactured for more than 40 years. A usual approach of filter designers has been to simply anti-reflect equal length cavity structures to the substrate and the exit medium. However, this yields filters with excessive ripple in the passband. In an attempt to obviate this problem, the need to modify the cavity lengths was investigated thoroughly by experts in the thin film field. P. W. Baumeister in a paper entitled "Use of microwave prototype filters to design multilayer dielectric bandpass filters", published in Applied Optics Vol. 21, No. 16, Aug. 15, 1982, describes the use of a standing wave ratio technique to match reflective zones applying microwave filter synthesis. C. Jacobs in an article entitled "Dielectric square bandpass design", in Applied Optics, Vol. 20, No. 6 Mar. 15, 1981, describes the use of an effective index approach. A. Thelen in a book entitled Design of Optical Interference Coatings, McGraw-Hill Book Company 1989, describes equivalent layers and other schemes to reduce ripple. However, the layer sequences developed from these methods are not considered to be general and do not apply equally well for different ratios of index of refraction of the multilayers.
Generally, procedures that result in high transmission reduce the bandwidth of this transmission at the expense of poor slopes (i.e. a slow rate of change) in the transition to blocking. Since the improvement in transmission outweighs the change in slope, additional cavities may be added to a filter to improve the slope.
In view of the limitations of the prior art, it is an object of this invention to provide a bandpass filter that overcomes many of these limitations.
Furthermore, it is an object of this invention to provide a bandpass filter in the form of a multi-layer, multi-cavity structure that reduces transmission ripple in the passband normally associated with other bandpass filters.